Interpretive Summary: Rift Valley fever (RVF) virus causes severe disease and death in domestic animals and humans in Africa and Arabia. The disease is transmitted by mosquitoes which are produced during periods of heavy rainfall. We have developed a technique whereby satellite sensors can be used to detect conditions of high rainfall that precede disease cases. In May and June 2005 we observed environmental conditions along the Red Sea on the Arabian Peninsula that might indicate that RVF disease transmission would occur. This information can be used by agricultural and public health officials to enhance disease surveillance and prepare to disease control operations.

Technical Abstract:
Rift Valley fever (RVF) was first confirmed outside of Africa in September 2000. This outbreak, which occurred in southwestern coastal Saudi Arabia and neighboring coastal areas of Yemen, followed elevated rainfall levels in nearby highlands which flooded the coastal areas, providing ideal environments for mosquito populations similar to those found in RVF endemic regions of East Africa. Satellite-derived rainfall estimates show that above-average rainfall occurred over most of western Saudi Arabia and Yemen from mid-April to mid-June 2005 and accounts for the high magnitude and spatial pattern of observed NDVI anomalies in May and June 2005. Rainfall was concentrated in the mountainous regions east of the Red Sea coast, and was heaviest in the areas east of Jeddah and Jizan, and in southwestern Yemen. In the area east of Jeddah, total rainfall in April 2005 was 150 mm above the long-term average for that month. Flooding was reported in Hodeidah governorate, Yemen during May and could be expected in other Red Sea coastal areas following such heavy rainfall, creating habitats appropriate for mosquitoes capable of transmitting RVF as occurred in 2000. No human cases have been reported in Saudi Arabia and Yemen since the 2000 outbreak, but in September 2004 the Saudi Ministry of Agriculture reported that 5 RVF seropositive sheep had been identified during routine surveillance in Jizan; primary infection was estimated to have occurred in April 2004. Since RVF virus can be maintained in mosquito eggs for extended periods and be transmitted under favorable conditions, the high magnitude of NDVI and rainfall patterns described above should prompt heightened veterinary and human surveillance for RVF in coastal Arabia and mass vaccination of susceptible animals if indicated based on surveillance.